This invention relates to generally to the field of discrete component shunt voltage regulator circuits. More specifically, it relates to a shunt voltage regulator circuit suitable for high voltage, low current applications, especially in high temperature environments.
Shunt voltage regulators are components or circuits that are usually connected in parallel with a particular electronic device, or across the input or output terminals of a circuit, to limit the voltage that can be applied across the device or between the terminals. The shunt regulator performs this function by conducting very little current until a preset voltage is reached, at which point the regulator becomes a very low resistance device that conducts a high current.
A well-known type of shunt voltage regulator is the zener diode. A zener diode exhibits a very high resistance, and thus allows the passage of very small currents, until a predefined reverse threshold voltage (or "zener" voltage) is applied across it. When the zener voltage is reached or exceeded, the zener diode becomes conductive with a variable current at the zener voltage. Zener diodes are commonly available with zener voltages of about 2 volts to about 400 volts. A problem with zener diodes is that those with zener voltages above about 5 or 6 volts exhibit large positive temperature coefficients (expressed in V/.degree. C.), as shown in the graph of FIG. 1. Thus, high voltage zener diodes are not suitable in many applications in which high ambient temperatures may be experienced. Of course, a large number of low voltage zener diodes may be connected in series to provide a high voltage regulator that is relative temperature-stable, but this is usually impractical in terms of cost and space considerations.
U.S. Pat. No. 5,949,122--Scaccianoce discloses an integrated circuit that provides thermal compensation for a series string of zener diodes, in which several bipolar transistors are connected as V.sub.BE multipliers. While this circuit provides temperature-stable high voltage regulation, it may not work well at very low collector currents. This is because the bipolar transistors are connected in a common emitter configuration, in which the collector current (I.sub.C) in each transistor is equal to the base current (I.sub.B) multiplied by the common emitter gain (H.sub.FE) of the transistor. The value of H.sub.FE for a typical bipolar transistor is in the range of about 10 to about 200. Since the collector current in the Scaccianoce device is the shunt regulation current, the base current would be between 0.5% and 10% of the shunt regulation current. Thus, at low shunt regulation currents (i.e., about 25 .mu.amps to about 500 .mu.amps), the base current would be at or near the value of the collector cutoff current (the collector-to-base leakage current, or I.sub.CBO) for typical bipolar transistors. There are bipolar transistors with values of I.sub.CBO low enough to allow the Scaccianoce device to work at low shunt regulation currents, but the value of I.sub.CBO exhibits a large positive temperature coefficient, especially at temperatures above about 100.degree. C. Thus, as a practical matter, a device constructed in accordance with the Scaccianoce disclosure to operate at low shunt regulation currents would be limited to operation in temperatures below about 125.degree. C.
The prior art also includes a gas discharge tube device that operates in the corona mode of discharge. This device operates as a high voltage equivalent of a zener diode, and it functions well with low shunt regulation currents and at high temperatures (100.degree. C. to 200.degree. C.). These devices are fragile, however, and expensive, and they require a radioactive component (a beta emitter), which may present a health concern in some contexts.
There is thus a need for a high voltage regulation device that can operate with low shunt regulation currents in high temperature environments. There is a further need for a device that meets these operational criteria, and that may also be realized in a space-efficient and shock-resistant package.